It means (I think) that the trough between the "good answer" and the "bad answer" is too big. So basically either you know it or not and can't "grow" toward a solution easily

I hate this kind of problems.

I think it can be summed up to; you're expecting a specific final solution. Open ended questions that don't have a best solution will require you to pay more attention to the process, rather than the result. In this article, the guy was smart enough to figure it out, but during the pressure of the interview, didn't stumble upon the right solution or fully verify his initial solution, but was able to as soon as the pressure was off. With an open ended solution it's not about getting it right, but how you try to come up with an answer.

I think this explains it.

http://en.wikipedia.org/wiki/Eureka_effect

> Is there a definition of ah-ha moment?

Not an exact definition, no. But if a question is exceedingly simple after one tiny bit of insight that doesn't require much analytical thinking to arrive at, that certainly falls into the category.